The present invention relates to the field of multilayer semiconductor structures or wafers produced according to the three-dimensional (3D) integration technology for transferring, onto a first wafer, called the final substrate, at least one layer formed from a second wafer, this layer corresponding to that portion of the second wafer in which elements, for example a plurality of microcomponents, have been formed, it being possible for the first wafer to be a virgin wafer or to comprise other corresponding elements.
In particular, because of the very small size and the large number of microcomponents present on a given layer, each transferred layer, that is to say each wafer comprising the layer, must be positioned on the final substrate (the first wafer alone or already having other transferred layers) with a correct precision so as to meet an alignment tolerance of around 0.3 microns with the subjacent layer. Furthermore, it may be necessary to carry out treatments on the layer after it has been transferred, for example to form other microcomponents, to expose, on the surface, microcomponents, to produce interconnects, etc., these treatments also having to be carried out very precisely with respect to the elements present in the layer.
These elements, such as microcomponents, are typically formed by the well-known technique of photolithography which consists mainly in irradiating a substrate made photosensitive (for example by applying a photoresist on the substrate) in defined zones corresponding to the locations where the microcomponents have to be formed. The irradiation of the substrate is typically carried out using a selective irradiation apparatus, commonly referred to as a “stepper”, which, unlike an apparatus for overall irradiation, irradiates during an operation only part of the substrate through a mask formed from opaque and transparent zones for defining the pattern that it is desired to reproduce on the substrate. The irradiation tool or stepper repeats the irradiation operation at as many places as necessary in order to irradiate the entire surface of the substrate.
The transfer of a layer onto the final substrate involves bonding, for example by direct bonding (also called molecular adhesion), between a first wafer and a second wafer of the type described above, the second wafer then being in general thinned. During bonding, the two wafers are mechanically aligned. At least three principle types of deformation resulting in alignment defects may be observed between the two wafers, namely deformations of the offset or shift type, deformations of the rotation type and deformations of the radial type (also known as run-out deformations, corresponding to a radial expansion that increases linearly with the radius of the substrate).
In general, the stepper is capable of compensating for these types of defect using a compensating algorithm. It has been found, however, that after transfer, cases exist in which it is very difficult, if not impossible, to form supplementary microcomponents aligned with respect to the microcomponents formed before the transfer, while respecting the microcomponent technology requirements, despite the use of such compensating algorithms.
In addition to alignment defects of the shift, rotation and radial type that are described above, inhomogeneous deformations may in fact occur in the transferred layer, because it is bonded by direct bonding, and also in the first wafer.
Now, it is these inhomogeneous deformations of the wafers that then result in this misalignment phenomenon, also called “overlay”, which is described in relation to FIG. 1. The overlay takes the form of defects of around 50 nm in size, these being markedly smaller than the alignment precision of the wafers at the moment of bonding.
FIG. 1 illustrates a three-dimensional structure 400 obtained by low-pressure direct bonding between a first wafer or initial substrate 410, on which a first series of microcomponents 411 to 419 have been formed by photolithography by means of a mask for defining the pattern formation zones corresponding to the microcomponents to be produced, and a second wafer or final substrate 420. The initial substrate 410 has been thinned after bonding so as to remove a portion of material present above the layer of microcomponents 411 to 419 and a second layer of microcomponents 421 to 429 has been formed on the exposed surface of the initial substrate 410.
Despite using positioning tools, however, offsets occur between certain of the microcomponents 411 to 419 on the one hand, and microcomponents 421 to 429 on the other, such as the offsets Δ11, Δ22, Δ33, Δ44  indicated in FIG. 1 (corresponding to the observed offsets between the pairs of microcomponents 411/421, 412/422, 413/423 and 414/424 respectively).
These offsets do not result from individual transformations (translations, rotations or combinations thereof) that could stem from an imprecise assembly of the substrates. These offsets result from inhomogeneous deformations that appear in the layer, coming from the initial substrate while it is being bonded to the final substrate. In fact, these deformations cause non-uniform, local displacements at certain microcomponents 411 to 419. Thus, certain microcomponents 421 to 429 formed on the exposed surface of the substrate after transfer exhibit variations in position with these microcomponents 411 to 419 that may be of the order of a few hundred nanometers, or even a micron. This misalignment or overlay phenomenon may make it impossible to use the stepper if the amplitude of the overlay after correction is still for example between 50 nm and 100 nm, depending on the application. It is therefore very difficult, if not impossible, to form supplementary microcomponents in alignment with the microcomponents formed before the transfer.
This overlay effect between the two layers of microcomponents may furthermore be a source of short circuits, distortions in the stack, or connection defects between the microcomponents of the two layers. Thus, in the case in which the transferred microcomponents are images formed from pixels and the purpose of the post-transfer processing steps is to form colour filters on each of the pixels, a loss of colouring function for certain of these pixels is observed.
Therefore, if this misalignment or overlay effect is not controlled, it thus results in a reduction in the quality and the value of the multilayer semiconductor wafers that are fabricated. The impact of this effect becomes increasingly critical because of the ever increasing requirements with respect to miniaturization of the microcomponents and the integration density per layer thereof.
Accordingly, there is a need in the art for improvements in this area, and such improvements are now provided by the present invention.